Air sterilizing plant

ABSTRACT

Air sterilizing plant for supplying completely sterile air which includes in the heating path of the air stream a filter means heated to the air sterilization temperature to assure uniformity of temperature in the air stream.

Waited States Patent [1 1 lEgger [111 3,858,645 [451 ,Ian. 7, i975 l l AIR STERILIZING PLANT [76] Inventor: Hermann Egger, CPI-1717 St. Ursen (FR), Switzerland [22] Filed: Mar. 23, 1972 [21] Appl, No.: 237,261

[30] Foreign Application Priority Data Apr. 1, 1971 Italy 49461/71 [52] US. Cl 165/66, 165/ll9, 21/53 [51] Int. Cl. A23c 3/02 [58] Field of Search 165/65, 66, 119; 21/53, 2l/94 R [56] References Cited UNITED STATES PATENTS 2,501,960 3/1950 Olson 165/66 3,254,942 6/l966 Harger 165/65 3,315,737 4/1967 Welch 165/1 19 3,737,626 6/l973 Davis et al l65/l 19 Primary ExaminerCharles Sukalo Attorney, Agent, or Firm-Browdy and Neimark [57] ABSTRACT Air sterilizing plant for supplying completely sterile air which includes in the heating path of the air stream a filter means heated to the air sterilization temperature to assure uniformity of temperature in the air stream.

6 Claims, 9 Drawing Figures PAIENTED 71975 3.858.645

SHEET 30F 5 AIR STERILIZING PLANT The present invention concerns an air sterilizing plant, which is capable of supplying completely sterile air for many uses, such as to keep in an aseptic condition all types of food processing, preserving and packaging apparatus and equipment and to protect against bacterial contamination precedently sterilized perishable products and materials which have to come into contact with said products.

It is known that air sterilization forms an essential problem in many fields of engineering, and particularly in the packaging industry. One known method of air sterilization was that of subjecting the air simultaneously to the combined action of heat and a chemical agent, such as vaporized hydrogen peroxide. This combined effect was intended to destroy the microorganisms contained in the air. The use of hydrogen peroxide presents considerable drawbacks, such as that of pipe corrosion, the correct dosage of the hydrogen peroxide, and finally some troubles, such as irritation of the eyes and of the mucosae of the machine personnel caused by the evaporation of this agent.

There are also known methods to sterilize air by merely heating it to temperatures where all microorganisms in it are destroyed. However, they fail to heat all the regions of the air stream flowing through them to the same high temperature required to sterilize them, and therefore they do not warrant a complete sterilization of this medium. An increase of heting power in such an equipment does not completely eliminate this deficiency, while, on the other hand, it considerably increases the air sterilization costs.

It is therefore the object of the present invention to provide an air sterilization plant which ensures a thorough sterilization of the air treated in it, is inexpensive to manufacture, easy to install, to operate and to maintain.

According to an essential .feature of the invention, this uniformity of temperature in the air stream is attained by interposing in the path of said air stream a heated metal filter. As a result, complete sterilization of the air stream may be attained even at temperatures which are consideraly lower than those which would be needed without the use of a heated filter.

The two embodiments of the invention will now be described, with reference to the attached drawings, for a purely exemplificative and in no way limitative purpose.

In the drawings:

FIG. 1 is a schematic illustration of the air sterilization plant according to the invention;

FIG. 2 is a diametral sectional view of the heating device of the plant according to FIG. 1;

FIG. 3 is a horizontal sectional view along line III-III of FIG. 2;

FIG. 4 is a diametral sectional view of a modified air heater;

FIG. 5 is a cross-sectional view along line V-V of FIG. 4;

FIG. 6 is a cross-sectional view along line VI-VI of FIG. 4;

FIG. 7 is a longitudinal sectional view along the heat exchanger of the plant shown in FIG. 1;

FIG. 8 is a cross-sectional view along line VIIIVIII of the device shown in FIG. 4;

FIG. 9 is a diagram of the thermal cycle of the air treated with the plant according to the present invention.

In the air sterilization plant shown in FIG. 1, a blower 1 delivers air under pressure through a line 8 to an air filter 2. The air filtered in 2 passes through a line 9 into the heat exchanger indicated at 3, where it is pre-.

heated, and flows through line 10 into the heater generally indicated at 4 and provided with heating elements 5. As it will be described with greater detail with reference to FIGS. 2 through 6, the air is uniformly heated in this device to a temperature at which all microorganisms are killed. The heated and sterilized air returns from the device 4 through line 11 into the heat exchanger, where it transfers part of its heat in counterflow to the air stream from line 9, issues from the device 3 through line 12, is cooled at 6 by the cooling circuit 7 to the desired operating temperature and is led through line 13 to its point of utilization.

As shown in FIG. 9, the thermal levels of the air passed through said sterilization plant are the following: the air passes through filter 2 and enters the heat exchanging and recovering device 3 at room temprature, is pre-heated in said device 3 to approximately C, is heated at 4 to 300C (which temperature warrants the total destruction of the microorganisms contained therein). This completely sterile air enters the device 3 at approximately 210C and issues therefrom at a temperature of C, to be ultimately cooled in 6 to the temperature at which it is to be utilized.

The heating device 4 by which the air stream is uniformly and thoroughly heated to its complete sterilization essentially consists of heating elements and a heated metal filter. The version shown in FIGS. 2 and 3 comprises a cylindrical shell 14 having an air outlet 16 for the sterilized air issuing into line 11. The cylindrical shell is closed at one end by a bottom plate 17 provided with an inlet 15 connected with line 10 for the preheated air from said heat exchanger, and at its opposite end by a top plate 18, supporting heating elements. The heating elements consist of two groups of electrical resistors, the first being a central group 5' surrounding the inlet 15 and the other a periphral group 5", said resistors being all carried by the above mentioned top plate 18. A cylindrical filter 19, made of sinterized stainless steel, surrounds the second heating group 5" and is held in place, eccentrically with respect to the shell 14, by a circular projection 17' of the bottom plate and a circular projection 18' of the top plate. This arrangement affords the filter a certain freedom of movement with respect to the other parts of the heating device, to allow for its different coefficient of expansion. The shell 14 is welded to the bottom plate, while its upper rim forms an outward bent flange, by which the shell is bolted to the top plate with the interposition of a seal.

Owing to this arrangement, the pre-heated air coming from line 10 to the interior of shell 14 through the inlet, flows axially along the first group 5 of resistors; successively, the air flow is forced to pass radially through the resistor group 5", these resistors being arranged so as to form a grid and to oblige the air to flow along a path during which it is heated to a high temperature. Filter 19 is brought by the resistors and the air stream uniformly to the sterilization temperature reigning within the heater, so that all the zones of the air stream, during their passage through the pores of the filter, assume the same temperature, say 300C. This heating and sterilizing effect is enhanced owing to the prolongation of the direct contact between the air and the hot metal surfaces. The inwardly facing surfaces of the circular projections 17' and 18 are concave, so as to bend the flow of air through the filter upward and downward respectively, in order to deflect it away from the lower and upper rim respectively of said filter in order to prevent the heated air to pass through the inerstices existing between the upper and lower rims of said filter and the lateral surfaces of the projections 17' and 18'. As already mentioned, these interstices must be left in order to compensate for the differences in radial and axial expansion between the filter and the remaining components of the heating device 4.

Altogether, the combination of the two above illustrated features, i. e. the air flow which is initially axial and thereafter radial through a grid of resistors (through which grid the air is forced into a turbulent motion) and of the filtration of said air through a very hot filtering wall, provides a very uniform heating of the air, which warrants therefore the best conditions to obtain its complete sterilization.

The heater shown in FIGS. 4, 5 and 6 comprises a first cylindrical shell 114, one end of which is closed by a plate 118, which also acts as a support for a central group of circularly arranged resistors 105' and a group of resistors 105 arranged in a number of circles concentric with the first group and surrounding it.

The other end of shell 114 is fastened to an annular plate 130 which closes one end of a second cylindrical shell 131. The opposite end of this shell is closed by a plate 117. In this embodiment the cylindrical metal filter is contained in this second shell 131 and supported between its end plates 130 and 117.

A hollow metal cylinder 132, surrounding the first central group of resistors 105', extends, coaxially with shells 114 and 131, from the internal rim of annular plate 130 to the vicinity of plate 118, to form a baffle separating the central group of heaters from the peripheral ones. The inlet 115 connected to the line 10 of the pre-heated air is applied to shell 114 adjacently to plate 130. The outlet 116, which is connected to the line 111 of sterilized air is applied to shell 131 adjacently to plate 130. Owing to this arrangement, the pre-heated air from line 10 passes through inlet 115 into the space between shell 114 and cylinder 132 and thence through the interior of the latter, during which path it is heated by the resistor groups 105 and 105' to a sterilizing temperature of approximately 300C. From cylinder 132 the air passes into space 133 delimited by filter 119 and plates 117 and 130, and thence through the pores of said filter into the space 134 located between said filter 119 and shell 131, to issue finally through the outlet 116 into line 11. In this process, the metal filter assumes uniformly the temperature of the sterilized air with which it is in contact and once it has reached this temperature, it performs its function to warrant the uniformity of the temperature of the air stream passing from space 133 into space 134.

Best results were obtained with sinterized steel filters having a mean porosity of 32 ,u. A test series based on the method of atomizing at the outlet of the heat exchanger a solution of thermoresistant spores of Bacillus subtilis and recovering the surviving spores correspondingly to the outlet of the plant (at the end of line 13) has shown that the use of the heated filter permits to obtain, even with air heated to 280C, the same survival index of the spores as that which is only obtainable when, without the use of a filter, the air is brought to 330C. At equal temperatures, the use of the heated filter assures a higher sterility of the air than that which is obtained without a heated filter.

The filter is easily regenerated by steam or by solvents.

The heat exchanging and recovering device illustrated in FIGS. 7 and 8 is also made with the purpose of providing a highly efficient piece of equipment which is very easy and unexpensive to manufacture, to instal and to operate. It comprises an inner casing 21 having a rectangular cross section, provided with an inlet 26 for the sterile air coming from line 11 and with an outlet 27 connected with line 12. The central portion of said casing is flatter and narrower than its two terminal portions, in order to obtain a practically laminar air stream passing therethrough.

A shell is welded to the top, and an additional shell is welded to the lower surface of said casing. Each shell 22 forms, together with the surface to which it is welded, a generally rectangular passage for the air to be pre-heated, having an inlet 24 connected with the line 9 and an outlet connected with the line 10. Correspondingly to casing 21, also the height of the central portions of these passages is reduced, in order to also obtain a practically laminar air stream and thus a more uniform pre-heating of the air passing from inlets 24 through the outlets 25. Thus, the configuration of this heat exchanging and recovering device ensures laminar and turbulent air streams both in the interior of the casing 21 and in the passages 22, with the air in casing 21 in counterflow with that in casing 22, in order to obtain the best possible heat exhange.

Unit 3 is insulated against the exterior by an outer casing 28.

FIG. 9 shows the time-temprature curve of air treated in a sterilization plant according to the invention (A). The numerals above each curve section correspond to the reference numerals of the single units wherein the various stages of the thermal cycle are performed. Thus the air enters at C into the heater, is heated to 300C, issues at this temperature level from filter 19 or 1 19, is cooled to C in the air exchanger and further cooled to a desired temperature in 6.

What is claimed is:

1. Apparatus for sterilizing air by the use of high temperature, comprising:

an air sterilizer, means to deliver air to said air sterilizer, and means to remove sterilized air from said air sterilizer;

said air sterilizer comprising means to destroy all micro-organisms in the air within said air sterilizer by providing an elevated temperature;

said means to destroy micro-organisms comprising a microporous metallic filter and heating means to heat said microporous filter and the air within said air sterilizer to a temperature of about 280300C, said microporous filter being located downstream of said heating means; and

air cooling means downstream from said air sterilizer.

2. Apparatus in accordance with claim 1 further comprising a preliminary filter upstream from said air sterilizer;

a pre-heater comprising a heat exchanger having a first passageway between said preliminary filter and said air sterilizer, and a second passageway in heat exchange relationship with said first passageway downstream of said air sterilizer, wherein said means to remove sterilized air from said air sterilizer serves to pass the sterilized air back through said second passageway of said heat exchanger to pre-heat incoming air passing through said first passageway from said preliminary filter; and

said air cooling means being located downstream from said second passageway of heat exchanger.

3. Apparatus according to claim 1, wherein said air sterilizer comprises:

a shell provided with an air outlet;

a bottom plate closing one end of said shell and provided with an air inlet;

a top plate closing the opposite end of said shell, the

internal faces of both plates being concave;

said heating means comprising a central group of heating elements surrounding said air inlet and a peripheral group of heating elements surrounding said central group;

said microporous filter being cylindrical and surrounding said peripheral group of heating elements and interposed between said group and said shell.

4. Apparatus according to claim 1, wherein said air sterilizer comprises:

a first shell closed at one end and containing said heating means;

a second shell having an inwardly extending annular plate at one end, said shell having fastened at said one end to the opposite end of said first shell, and being closed at its other end;

a hollow metal cylinder communicating with said second shell and said first shell and extending from the internal rim of said annular plate and protruding into the interior of said first shell;

an air outlet in said second shell;

said microporous filter being interposed in said second shell between said heating means and said air outlet.

5. A plant according to claim 1 wherein the filter is a cylindrical sinterized metallic filter.

6. A plant according to claim 5 wherein said filter is sintered steel and the mean size of the pores of said filter is 32 u.

UNITED STATES PATENT AND TRADEMARK OFFICE Certificate Patent No. 3,858,645 Patented March 23, 1972 Hermann Egger Application having been made by Hermann Egger, the inventor named in the patent above identified, and International Paper Co., the assignee, for the issuance of a certificate under the provisions of Title 35, Section 256, of the United States Code, deleting the name of Hermann Egger and adding the names of Silvano Moscatelli and Pier Luigi Locchi as joint inventors, and a showing and proof of facts satisfying the requirements of the said section having been submitted, it is this 19th day of Mar. 1985, certified that the name of the said Hermann Egger is hereby deleted from the said patent as sole inventor and the name of the said Silvano Moscatelli and Pier Luigi Locchi are hereby added to the said patent as joint inventors.

Fred W. Sherling, Associate Solicitor. 

1. Apparatus for sterilizing air by the use of high temperature, comprising: an air sterilizer, means to deliver air to said air sterilizer, and means to remove sterilized air from said air sterilizer; said air sterilizer comprising means to destroy all microorganisms in the air within said air sterilizer by providing an elevated temperature; said means to destroy micro-organisms comprising a microporous metallic filter and heating means to heat said microporous filter and the air within said air sterilizer to a temperature of about 280*-300*C, said microporous filter being located downstream of said heating means; and air cooling means downstream from said air sterilizer.
 2. Apparatus in accordance with claim 1 further comprising a preliminary filter upstream from said air sterilizer; a pre-heater comprising a heat exchanger having a first passageway between said preliminary filter and said air sterilizer, and a second passageway in heat exchange relationship with said first passageway downstream of said air sterilizer, wherein said means to remove sterilized air from said air sterilizer serves to pass the sterilized air back through said second passageway of said heat exchanger to pre-heat incoming air passing through said first passageway from said preliminary filter; and said air cooling means being located downstream from said second passageway of heat exchanger.
 3. Apparatus according to claim 1, wherein said air sterilizer comprises: a shell provided with an air outlet; a bottom plate closing one end of said shell and provided with an air inlet; a top plate closing the opposite end of said shell, the internal faces of both plates being concave; said heating means comprising a central group of heating elements surrounding said air inlet and a peripheral group of heating elements surrounding said central group; said microporous filter being cylindrical and surrounding said peripheral group of heating elements and interposed between said group and said shell.
 4. Apparatus according to claim 1, wherein said air sterilizer comprises: a first shell closed at one end and containing said heating means; a second shell having an inwardly extending annular plate at one end, said shell having fastened at said one end to the opposite end of said first shell, and being closed at its other end; a hOllow metal cylinder communicating with said second shell and said first shell and extending from the internal rim of said annular plate and protruding into the interior of said first shell; an air outlet in said second shell; said microporous filter being interposed in said second shell between said heating means and said air outlet.
 5. A plant according to claim 1 wherein the filter is a cylindrical sinterized metallic filter.
 6. A plant according to claim 5 wherein said filter is sintered steel and the mean size of the pores of said filter is 32 Mu . 